Auxiliary battery sensor switch

ABSTRACT

An apparatus for sensing the presence of an auxiliary battery connected to a cellular telephone is disclosed. An internal battery and a removable auxiliary battery are connected to the power system of the cellular telephone through first and second switches respectively. Connection of the auxiliary battery creates a current flow that is detected by a current sensing circuit which generates a current signal indicating the detection of current flow. A voltage sensing circuit determines whether the auxiliary battery can provide a predetermined voltage level and generates a voltage signal indicating the results of this test. A processor then utilizes the signals from the voltage sensing circuit and the current sensing circuit to control the pair of switches to connect the main and auxiliary batteries to the power circuitry of the cellular telephone.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

The present invention relates generally to means for extending thebattery life of portable electronic devices such as cellular telephones,and more particularly, to a switch capable of sensing the presence of anauxiliary battery and switching the power circuitry of the electronicdevice to the auxiliary battery.

2. Description of Related Art

The battery pack is the main power source for portable electronicdevices such as cellular phones. The portable nature of cellular phonesare limited by the fact that their battery power will not last forever.After a period of time the batteries become discharged and the cellularphone must be plugged into a charger in order to recharge the battery.In order to achieve more battery power (longer battery life), typicallythe size of the battery cell must be increased. This presents a problemfor cellular telephone designers due to the conflicts between a desirefor longer talk time and standby for the phone (i.e., longer batterylife) and the desire for smaller, lighter telephones.

Existing solutions for the problems of shortened talk time and standbyhave been limited to the development of high capacity batteries forcellular telephone units. These batteries suffer from the limitations ofsize and weight mentioned previously. The conflict between size andpower is made worse by the development of next generation telephoneswhich are becoming smaller and smaller. Other solutions involve the useof improved battery technologies that increase the charge per volume andcharge per weight ratios of existing battery systems. However, theseimprovements have not been sufficient to keep up with the smaller sizesof cellular telephone units that are possible using existing componentsand technologies. Thus, what is necessary for meeting the presentlyexisting market needs is a means for increasing a cellular telephone'sbattery life while at the same time leaving the size and weight of thecellular telephone unchanged.

SUMMARY OF THE INVENTION

The present invention overcomes the foregoing and other problems with anapparatus for switching between a main and an auxiliary battery inresponse to connection of the auxiliary battery to an electronic devicesuch as a portable communications device. The main and auxiliarybatteries are connected to system power through first and secondswitches respectively. The main battery is located within the cellulartelephone unit but is only connected to system power when the auxiliarybattery is not connected or the auxiliary battery's voltage falls belowa particular level. The auxiliary battery is connected to the beltholder for the cellular telephone. The first and second switches arecontrolled by control signals from a microprocessor.

In a first embodiment, connection of the auxiliary battery creates acurrent through the switching circuitry which is detected by a currentsensor. The current sensor generates a current signal in response to thecurrent flow and provides this signal to the microprocessor. A voltagesensor measures the voltage level present within the auxiliary batteryto determine if the voltage level meets a predetermined thresholdvoltage. The voltage sensor then generates a voltage signal indicatingwhether or not the voltage level meets the desired threshold andprovides this signal to the microprocessor.

An alternative embodiment, connection of the auxiliary battery generatesa voltage across a resistor in the switching circuitry which is detectedby a voltage sensor. The voltage sensor generates a signal in responseto the voltage across the resistor and provides this to themicroprocessor. In response to detection of the voltage, the voltagesensor also measures the voltage level present within the auxiliarybattery to assist in determining if the voltage level meets apredetermined threshold level.

The microprocessor utilizes the current signal and/or voltage signal todetermine which battery to connect to system power. If the signalsindicate the auxiliary battery is not connected, the main battery isconnected. If the signals indicate the auxiliary battery is connectedbut the auxiliary battery does not contain sufficient voltage, the mainbattery is connected and the auxiliary battery is disconnected. However,if the auxiliary battery is connected and contains a sufficient voltagelevel, the auxiliary battery is connected while the main battery isdisconnected. In this manner the effective battery life of the mainbattery is extended without corresponding increases in the size of thebattery supply associated with the cellular telephone unit.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, reference ismade to the following detailed description taken in conjunction with theaccompanying drawings wherein:

FIG. 1 is a side view of a cellular telephone and a telephone holderincorporating an auxiliary battery;

FIG. 2 is an alternative embodiment of the invention disclosed in FIG. 1wherein the holder includes an opening for the attachment of additionaldevices;

FIG. 3 is a block diagram illustrating the circuitry for switchingbattery power between the auxiliary battery and the main battery when acellular telephone is placed in the telephone holder;

FIG. 4 is a circuit diagram of the current sensor block of FIG. 3;

FIG. 5 is a block diagram illustrating the control flow of themicroprocessor of FIG. 4;

FIG. 6 is an alternative embodiment of the circuitry of FIG. 3 forswitching power between the auxiliary battery and the main battery; and

FIG. 7 is a block diagram illustrating the control flow of themicroprocessor of FIG. 6.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring now to the Drawings, and more particularly to FIG. 1, there isillustrated a side view of a cellular telephone holder 15 incorporatingan auxiliary battery and an associated cellular telephone unit 10. Thecellular telephone unit 10 includes the main housing 20, a battery 25and antenna 30. Contact 35 at the base of the cellular telephoneprovides a connection to the power circuitry of the cellular telephoneunit 10. The circuitry interconnected with the contact 35 will be morefully discussed in a moment. While the present description illustratesthe contact being placed at the base of the cellular telephone unit 10,the contact may of course be located at any point enablinginterconnection of the cellular telephone unit 10 with the auxiliarybattery 40 of the telephone holder 15.

The cellular telephone holder 15 includes the auxiliary battery 40integrally connected with a housing 55. The holder housing 55 isdesigned such that it is integrated with auxiliary battery 40 as asingle piece or such that the auxiliary battery may be removablyconnected to the holder housing by sliding, snapping or some other typeof connection means. The cellular telephone holder 15 may also include acharger connection 49 for connecting a battery recharger.

The housing 55 defines a cradle 53 for holding the cellular telephoneunit 10. A holding rib 56 in the housing 55 may be positioned tolockably engage a corresponding slot 60 within the housing 20 of thecellular telephone unit 10. The rib 56 and slot 60 combination securethe telephone unit 10 within the cradle 53 while still enabling removalof the telephone unit from the telephone holder 15. A clip 45 forenabling a user to attach the cellular telephone holder 10 to someportion of their clothing, most likely their belt, is connected to theback of the holder housing 55. This enables the weight of the auxiliarybattery 40 to be more conveniently carried in a place other than thetelephone unit 15.

The cradle 53 for the cellular telephone unit 10 further includes acontact 70 for engaging the contact 35 of the portable telephone unit10. The contact 70 interconnects the positive and negative terminals ofthe auxiliary battery 40 to the portable telephone unit 10.

Internal to the portable telephone unit 10 is auxiliary battery sensorcircuitry 80 for switching system power between the main 25 and theauxiliary 40 batteries. Switching between the batteries is responsive tochanges in current initiated by placement or removal of the telephoneunit 10 from the housing 55. The sensor 80 also switches power back tothe main battery 25 should the charge of the auxiliary battery 40 becomedepleted. The preferred embodiment of the sensor circuitry 80 will bemore fully discussed with respect to FIGS. 3-5.

Referring now to FIG. 2 there is illustrated a side view of analternative embodiment of the cellular telephone holder 55 wherein thereis included within the cradle, an opening 85 enabling the attachment ofperipheral devices to the telephone unit 10. Through this opening 85data accessories, chargers, and/or alternative telephone connections maybe connected to the telephone unit 10.

It should be realized that while FIGS. 1-2 have been described withrespect to the use of a cellular telephone and cellular telephoneholder, the use of the electronic holding device having an integratedauxiliary battery would be equally applicable to any electronic devicehaving a standby mode and an internal battery or a need to increasebattery life.

Referring now to FIG. 3, there is illustrated the auxiliary batterysensor. circuitry 80 for detecting the connection of a auxiliary battery40 to the cellular telephone unit 10. The sensor circuitry 80 incudes apair of connections 100 for interconnecting the main battery 25 andauxiliary battery 40 to the power circuitry of the telephone unit 10. Apair of switches 115 and 120 interconnect the main 25 and auxiliary 40batteries to the remainder of the telephone power circuitry 118. Theswitches 120 and 115 are controlled by a microprocessor 125 in responseto control signals received from a current sensor 130 and a voltagesensor 135.

The current sensor 130 detects current passing through a resistor 150.Current flow through the resistor 150 is initiated by connection of theauxiliary battery 40 to the portable telephone unit 10. Referring nowalso to FIG. 4 wherein there is illustrated the circuitry of the currentsensor 130. Voltage readings (V₁ and V₂) are taken from each end of theresistor 150 and applied to the inputs of a pair of operationalamplifiers 155. The voltage V₁ is connected to the positive input ofoperational amplifier 155a while voltage V₂ is connected to the negativeinput. The inputs of the operational amplifier 155b are connected to thevoltages in the opposite manner. The operational amplifiers 155 amplifythe difference in voltage across the resistor 155. Since a negativesupply voltage is not available within the portable telephone unit 10,each operational amplifier 155 has its negative supply 162 grounded andcannot supply a negative voltage. Consequently, two operationalamplifiers 155 are needed to amplify the differential voltage for eachdirection of current flow.

The voltage differential output of each operational amplifier 155 isprovided to the inputs of an OR gate 170. If either the first or secondoperational amplifier 155 generates a high voltage, indicating currentis flowing through the sensor resistor 150, the OR gate 170 outputs alogical high voltage signal. This is interpreted by the microprocessor125 as connection of a auxiliary battery and switch 120 is closed whileswitch 115 is opened to preserve power to the main battery 25. When theoutput of both the first and second operational amplifiers 155 are low,OR gate 170 generates a logical low voltage signal, and switch 115 isclosed to provide power from the main battery 25. Switch 120 alsoremains closed to enable current flow to occur through resistor 150 oncean auxiliary battery 40 is connected.

Referring back to FIG. 3, the voltage sensor 135 determines the voltageoutput of a connected auxiliary battery 40 and generates a voltagesignal indicating whether sufficient voltage exists to power thecellular telephone unit 10. Sufficient voltage is based upon whether theauxiliary battery 40 can supply a pre-selected threshold voltage. Whenthe auxiliary battery voltage falls below the predetermined threshold, asignal is generated indicating this to the microprocessor 125. Switch120 then opens to disconnect the auxiliary battery 40 from the powercircuitry 118, and switch 115 closes to connect the main battery 25.With switch 120 open resistor 150 can no longer detect connection of theauxiliary battery 40. Therefore, a resistor 200 enables detection of aphysical connection of the auxiliary battery 40 to connection 100. Whenthe auxiliary battery 40 engages connector 100, the voltage acrossresistor 200 will be positive. Once the auxiliary battery 40 isdisconnected, the voltage across resistor 200 goes to zero. The voltageacross resistor 200 is monitored by the voltage sensor 135.

Referring now to FIG. 5 there is illustrated a flow diagram describingthe algorithm for controlling switches 115 and 120 by the microprocessor125. This algorithm outlines the decision processes followed by themicroprocessor 125. Initially, both switches 115 and 120 are closed.Inquiry step 210 determines whether an auxiliary battery 40 has beenconnected to the cellular telephone unit 10 by measuring current throughresistor 150. The microprocessor 125 continues to monitor for connectionof the auxiliary battery 40 at step 210 until a connection is detected.Once an auxiliary battery 40 connection is detected, inquiry step 215determines if the auxiliary battery voltage is too low. When the voltagelevel is to low, switch 120 is opened at step 220. Inquiry step 221determines whether the voltage across resistor 200 is greater than zero.As mentioned previously, a voltage across resistor 200 of greater thanzero indicates that the auxiliary battery remains connected. If theauxiliary battery 40 is connected, control passes back to step 215 todetermine whether or not the battery voltage is too low. If theauxiliary battery 40 is disconnected, switch 120 is closed at step 222and control passes back to step 210 to monitor for a re-connection ofthe auxiliary battery.

If sufficient voltage exist within the auxiliary battery 40, switch 115is opened at step 225. Inquiry step 230 determines whether the auxiliarybattery 40 is still connected by sensing current through resistor 200.If the auxiliary battery 40 remains connected, inquiry step 235determines if sufficient auxiliary battery voltage still remains. Aslong as the auxiliary battery 40 remains connected and providessufficient voltage, control continues to loop through inquiry steps 230and 235 to monitor for auxiliary battery 40 disconnection and lowvoltages. Should the voltage within the auxiliary battery 40 drop belowacceptable levels, switch 120 is opened at step 240 while switch 115 isclosed at step 245 to connect the telephone unit 10 to the main battery25.

Once the main battery 25 has been reconnected, inquiry step 246determines whether the auxiliary battery 40 is still connected bydetermining whether the voltage across resistor 100 is greater thanzero. If so, control passes back to step 235 to determine whether or notthe auxiliary battery voltage is still to low. If the auxiliary batteryhas been disconnected, control passes to steps 250 and 255 to resetswitches 120 and 115 to await reconnection of the auxiliary battery 40.

Once the auxiliary battery 40 is disconnected, switch 115 is closed atstep 250 while switch 120 is closed at step 255. This passes control ofsystem power to the main battery 25 and control returns back to step 210to wait for reconnection of the auxiliary battery 40.

Using the circuitry and algorithm described above, the apparatus willoperate in the following manner. In normal operation, the auxiliarybattery 40 is not connected, and the power source for the telephone unit10 is the main battery 25. Switch 115 is closed so that the phone drawscurrent from the main battery 25. Switch 120 also remains closed toenable detection of the connection of an auxiliary battery 40. When auser connects the auxiliary battery 40, current passes through thesensor resistor 150 and voltage across resistor 200 becomes greater thanzero. This current flow is detected by the current sensor 130 and acurrent signal notifying the microprocessor 125 is generated.

The voltage sensor 135 and microprocessor 125 then determine ifsufficient voltage exists to operate the phone unit 10. If the voltageof the auxiliary battery 40 is too low, the microprocessor 125 keepsswitch 115 closed and opens switch 120. This prevents the auxiliarybattery 40 from draining current from the main battery 25. If thevoltage is sufficient, the microprocessor 125 opens switch 115 toprevent further current drain from the main battery 25.

Switch 115 remains opened and switch 120 closed until such time as thevoltage of the auxiliary battery 40 drops below the preselectedthreshold level, or the telephone unit 10 is disconnected from theauxiliary battery. In either case, switch 115 is closed so that thetelephone unit 10 is now powered by the main battery 25. If switch 115is closed due to low voltage level switch 120 is opened for the reasonsdiscussed above. If the auxiliary battery 40 has been disconnected asindicated by the voltage across resistor 200, switch 120 remains closedto enable detection of current flow to the sensor resistor 150.

The above described circuitry has been described with respect to acellular telephone unit. However, it should be realized that thecircuitry is applicable to other electronic devices requiring increasedbattery times without an associated increase in device size.

Referring now to FIG. 6, there is illustrated an alternative embodimentof the present invention wherein only a voltage sensor 135 monitors forconnection of the auxiliary battery 40. Similar reference numerals areused for common parts as those described in FIG. 3. The sensor circuitry80 includes a pair of connections 100 for interconnecting the mainbattery 25 and auxiliary battery 40 to the power circuitry 118 of thetelephone unit 10. A pair of switches 115 and 120 interconnect the main25 and auxiliary 40 batteries to the remainder of the telephone powercircuitry 118. The switches 120 and 115 are controlled by amicroprocessor 125 in response to control signals received from avoltage sensor 135.

The voltage sensor 135 monitors the voltage across a resistor 200. Thevoltage across resistor 200 changes in response to connection of theauxiliary battery 40 to connector 100 of the sensor circuitry 80. Whenthe auxiliary battery 40 engages connector 100, the voltage acrossresistor 200 will be positive. Once the auxiliary battery 40 isdisconnected, the voltage across resistor 200 goes to zero if switch 120is open.

The voltage sensor 135 also monitors the voltage output of the connectedauxiliary battery 40 and generates a signal indicating whethersufficient voltage exists to power the cellular telephone unit 10.Sufficient voltage is based upon whether the auxiliary battery 40 cansupply a preselected threshold voltage. When the auxiliary batteryvoltage falls below the predetermined threshold, a signal is generatedindicating this to the microprocessor 125. The auxiliary battery is thendisconnected from the circuit.

Referring now to FIG. 7, there is illustrated a flow diagram describingthe algorithm for controlling switches 115 and 120 by the microprocessor125 for the embodiment of FIG. 6. The algorithm outlines the decisionprocesses followed by the microprocessor 125. Initially, switch 115 isclosed while switch 120 is opened. This connects the main battery 25 tothe telephone power circuitry 118. Inquiry step 300 monitors the voltageacross resistor 200 to determine whether or not the voltage is greaterthan zero. A voltage level of zero indicates that the auxiliary battery40 has not been connected and control continues to loop through step300. When connection of the auxiliary battery 40 is detected, inquirystep 305 determines whether the auxiliary battery voltage is too low. Ifso, control passes to inquiry step 310 to again determine if the voltageacross resistor 200 is greater than zero. If so, control continues toreturn to step 305. Once the voltage across resistor 200 indicates theauxiliary battery 40 has been disconnected, control passes back to step300.

If inquiry step 305 determines the auxiliary battery voltage is not toolow, switch 120 is closed at step 315 and switch 115 is opened at step320. This disconnects the main battery 25 from the power circuitry 118and connects the auxiliary battery 40. The voltage sensor 135 continuesto monitor for the connection of the auxiliary battery 40 to the powercircuitry 118 at inquiry step 325. If the resistor 200 maintains apositive voltage, inquiry step 330 again checks whether the auxiliarybattery voltage is too low. If so, switch 115 is closed at step 335 andswitch 120 is opened at step 340 to disconnect the auxiliary battery 40from the power circuitry 118 and control returns to step 325. If thevoltage is not too low, the processor 125 merely continues to monitorfor continued connection by returning to step 325.

Once inquiry step 325 determines the voltage across resistor 200 is nolonger greater than zero, switch 115 is closed at step 345 and switch120 is opened at step 350 to reconnect the main battery 25. Control thenreturns to step 300 to monitor for reconnection of an auxiliary battery40.

Using the circuitry and algorithm discussed above, the alternativeembodiment will operate in the following manner. In the normal mode ofoperation, the auxiliary battery 40 is not connected, and the source ofpower for the cellular telephone unit 10 is the main battery 25. Thus,in normal operation, switch 115 is closed so that the cellular telephoneunit 10 can draw current from the main battery 25. Switch 120 is openedduring normal operation. The cellular telephone unit microprocessor 125continually monitors the voltage sensor 135 to determine the voltageacross resistor 200.

When an auxiliary battery 40 is connected, a voltage drop is createdacross resistor 200 and the voltage of the auxiliary battery 40 can beread through voltage sensor 135. The microprocessor 125 then determinesif the auxiliary battery 40 has sufficient voltage to operate thecellular telephone unit 10. If the voltage of the auxiliary battery 40is too low to operate the cellular telephone unit 10, the microprocessor125 will do nothing and the phone will continue to be powered by themain battery 25. If the voltage of the auxiliary battery 40 is highenough, the microprocessor 125 will close switch 120, enabling thecellular telephone unit 10 to draw current from the auxiliary battery40. The cellular telephone unit 10 will then open switch 115 therebypreventing any further current drain from the main battery 25.

The cellular telephone unit 10 will keep switch 115 opened and switch120 closed until such time as the microprocessor 125 detects either thevoltage of the auxiliary battery 40 falling below the minimum thresholdlevel, or that the cellular telephone unit 10 has been disconnected fromthe auxiliary battery 40. In either case, once this is detected, switch115 closes so that the telephone can be powered by main battery 25.Switch 120 is opened to facilitate future detection of an auxiliarybattery 40 with sufficient voltage to power the telephone.

Although a preferred embodiment of the method and apparatus of thepresent invention has been illustrated in the accompanying Drawings anddescribed in the foregoing Detailed Description, it is understood thatthe invention is not limited to the embodiment disclosed, but is capableof numerous rearrangements, modifications, and substitutions withoutdeparting from the spirit of the invention as set forth and defined bythe following claims.

What is claimed is:
 1. An apparatus for switching between a main batteryand a removable second battery in an electronic device, comprising:meansfor interconnecting the main battery and the auxiliary battery to apower system of the electronic device; means for sensing a voltage dropinitiated by connection of the removable auxiliary battery to theelectronic device and for generating a voltage signal indicating apresence of the removable auxiliary battery; and a processor, responsiveto the voltage signal generated by the means for sensing, forinstructing the means for interconnecting to connect the auxiliarybattery to system power and disconnect the main battery from systempower.
 2. The apparatus of claim 1 wherein the means for sensingcomprises:a resistor connected to the electronic device such thatcurrent flows through the resistor in response to connection of theauxiliary battery; and a current sensor for generating the currentsignal indicating current flow in the resistor.
 3. The apparatus ofclaim 2 wherein the current sensor comprises:first and secondoperational amplifiers for measuring the differential voltage across theresistor in each current direction; and an OR gate responsive to theoutputs of the first and second operational amplifiers for generatingthe current signal indicating current flow in the resistor.
 4. Theapparatus of claim 3 wherein the signal further indicates removal of theauxiliary battery from the electronic device.
 5. The apparatus of claim4 wherein the processor is further responsive to the signal forinstructing the means for interconnecting to disconnect the auxiliarybattery from system power and connect the first battery to system power.6. The apparatus of claim 5 further including a voltage sensor fordetermining if the auxiliary battery provides a preselected voltagelevel and generating a voltage signal indicative thereof.
 7. Theapparatus of claim 6 wherein the processor disconnects the auxiliarybattery and connects the main battery when the voltage signal indicatesa auxiliary battery voltage below the preselected voltage level.
 8. Theapparatus of claim 7 wherein the means for detecting comprises means forsensing a voltage drop initiated by connection of the removeableauxiliary battery to the electronic device and for generating a voltagesignal indicating a presence of the removal auxiliary battery.
 9. Theapparatus of claim 1 wherein the means for sensing comprises:a resistor;and means for sensing a voltage change across the resistor initiated byconnection of the removable auxiliary battery to the electronic deviceand for generating a voltage signal indicating a presence of the voltagechange.
 10. An apparatus for switching between a main battery and aremovable auxiliary battery associated with a portable communicationsdevice, comprising:first and second switches, associated with the mainand the auxiliary batteries respectively, for interconnecting the firstbattery and the second battery to power circuitry of the portablecommunications device; a current sensing circuit for detecting a currentflow initiated by connection of the auxiliary battery and generating acurrent signal indicative thereof; a voltage sensing circuit fordetermining if the auxiliary battery provides a preselected voltagelevel and generating a voltage signal indicative thereof; and aprocessor for controlling the first and the second switches to connectand disconnect the main battery and the auxiliary battery in response tothe voltage signal and the current signal.
 11. The apparatus of claim 10wherein the current sensing circuit comprises:a resistor connected tothe portable communications device through which current flows inresponse to connection of the second battery; and a current sensor forgenerating the current signal in response to current flow through theresistor.
 12. The apparatus of claim 11 wherein the current sensorcomprises:first and second operational amplifiers for measuring thedifferential voltage across the resistor; and an OR gate responsive tothe outputs of the first and second operational amplifiers forgenerating the current signal indicating the current flow in theresistor.
 13. The apparatus of claim 10 wherein the processordisconnects the main battery and connects the auxiliary battery uponindication of current flow by the current signal.
 14. The apparatus ofclaim 10 wherein the processor connects the main battery and disconnectsthe auxiliary battery upon indication of no current flow by the currentsignal.
 15. The apparatus of claim 10 wherein the processor disconnectsthe second battery and connects the main battery when the voltage signalindicates a auxiliary battery voltage below the preselected voltagelevel.
 16. An apparatus for switching between a main battery and aremovable auxiliary battery associated with a portable communicationsdevice, comprising:first and second switches, associated with the mainand auxiliary batteries, respectively, for interconnecting the firstbattery and the second battery to power circuitry of the portablecommunications device; a resistor connected to the portablecommunications device across which a voltage drop occurs in response toconnection of the auxiliary battery; a voltage sensor for generating afirst voltage signal in response to the voltage drop across theresistor, said voltage sensor further determining if the auxiliarybattery provides a preselected voltage level in generating a secondvoltage signal indicative thereof; and a processor for controlling thefirst and second switches to connect and disconnect the main battery andthe auxiliary battery in response to the first and second voltagesignals.
 17. The apparatus of claim 16 wherein the processor disconnectsthe main battery and connects the auxiliary battery upon indication of avoltage drop by the first voltage signal.
 18. The apparatus of claim 16wherein the processor connects the main battery and disconnects theauxiliary battery upon indication of no voltage drop by the firstvoltage signal.
 19. The apparatus of claim 16 wherein the processordisconnects the auxiliary battery and connects the main battery when theauxiliary voltage signal indicates a second battery voltage below thepreselected voltage level.
 20. An apparatus for sensing the presence ofcurrent flow through a circuit, comprising:a resistor in series with thecircuit; first and second operational amplifiers for measuring thedifferential voltage across the resistor; and an OR gate responsive tooutputs of the first and second operational amplifiers for generatingthe signal indicating a current flow in the resistor.
 21. The apparatusof claim 20 wherein a negative voltage supply of the first and secondoperational amplifiers is grounded.
 22. An apparatus for switchingbetween a main battery and a removable auxiliary battery in anelectronic device, comprising:means for interconnecting the main batteryand the auxiliary battery to a power system of the electronic device; acurrent sensing circuit for detecting a current flow initiated byconnection of the auxiliary battery and generating a current signalindicative thereof; and a processor, responsive to the current signalgenerated by the current sensing circuit, for instructing the means forinterconnecting to connect the auxiliary battery to system power anddisconnect the main battery from system power.